One in 20 Cornwall residents said they were in poor health when asked in the 2021 census, new figures show.
The data shows wide disparities in health across England and Wales, with some areas having over three times more people in bad health than others.
Health think tank the Nuffield Trust said the postcode lottery of health tracks socio-economic deprivation, and called on the Government to address disparities in healthcare, provision and overall health guidance.
Office for National Statistics figures show 5.6% of residents in Cornwall said they were in "bad or very bad" general health in the 2021 census.
Nationally, 5.4% of people responded to the census saying they were in bad or very bad health.
This fell to 4.8% in the South West.
The Nuffield Trust said people living in certain areas fair much worse than others, and that the improvement in health across the country has not been enjoyed uniformly.
Sally Gainsbury, senior fellow at the think tank, highlighted the impact socio-economic factors had on people's health.
She said: "The role of socio-economic inequalities was often overlooked in health inequalities policy in the decade up until the pandemic. NHS England needs to follow through with their new focus on economic deprivation, as well as other, often related, drivers of health inequalities such as racial discrimination and social exclusion.
"We look forward to the Government giving these issues the attention they deserve in the long-delayed white paper on health inequalities."
There were also regional disparities in the number of unpaid carers across the country, the figures show.
In London and the South East, 4% of the population said they provided at least 20 hours of unpaid care per week. At the other end of the scale, this figure rose to 5.8% in Wales.
In Cornwall, 5% of the population said they provided high levels of unpaid care.
The data also shows 8.1% of Cornwall residents were classed as disabled in 2021.
By the ONS ranking of health levels – which takes into account poor health, disability and unpaid care – Cornwall ranks 222nd of England and Wales's 331 local authority areas.
The Department for Health and Social Care said the gap in the number of years people live in good health is "stark and unacceptable".
A spokesperson added: "We recently announced a major conditions strategy to address regional disparities in health outcomes, supporting the levelling up mission to narrow the gap in healthy life expectancy by 2030.
"There is already work ongoing across the North East, with the local Integrated Care Board investing £39 million over the next three years to prevent ill health and address health disparities."